The present invention relates to mechanical pencils.
Mechanical pencils are known in which a lead is advanced along a lead-advancing passage formed in a tubular casing. The tubular casing further includes a collet movable for advancing the lead between a closed position in which the lead is clamped and an open position in which the lead is released. The casing has a tip movable relative thereto and projecting normally outwardly away from the casing.
When the collet is open, the released lead tends to fall out of the casing, for example, due to the force of gravity should the pencil be in a substantially vertical position. Moreover, when the collet moves back from the open into the closed position, it slides along the lead. The friction between the slidable collet and the lead may cause a retracting movement of the latter together with the collet in direction inside the casing.
It has been suggested to provide the mechanical pencil with an elastic braking element surrounding the lead immediately adjacent the tip, for preventing the above-described undesired movements (i.e. slip) of the lead relative to the casing. Such a braking element frictionally engages the lead so as to develop a friction force sufficient to prevent the lead from falling out of the casing when the collet is open and eliminate any retracting movement of the lead together with the collet inwardly into the casing when the collet moves from the open into the closed position. However, such a friction force must be small enough to permit advancement of the lead when the collet moves from the closed towards the open position.
The braking element may be movable together with the tip which is movable axially inwardly into the casing as the lead is used up during writing or drawing. The braking element uniformly surrounds and supports the lead.
The tip may be urged into its ultimate forward position either by an end face of the collet or by a spring, so that the braking element moves the lead slightly forward. In this case the braking element has the additional function of moving the lead in direction outwardly of the casing.
It is known to provide a braking element of elastic synthetic plastic material, for example, rubber, which has an annular cross-section to encompass the lead and to develop friction between the inner surface of this braking element and the lead.
Such a braking element is, for example, described in German Pat. No. 18 15 535 and has a number of disadvantages.
The soft elastic synthetic plastic material of the braking element is very sensitive to abrasion (i.e. wear). This is especially true in the case of a very small contact surface between the braking element and the lead. Moreover, in time a braking element of such material changes its characteristics which leads to unpredictable and, therefore, uncontrollable changes in the friction engagement between the braking element and the lead. Such braking elements are produced in series (i.e. in great quantities); therefore, the characteristics of the braking elements, stipulated by the small size of the braking element and by shrinking of the material thereof, vary considerably.
Due to manufacturing considerations, the wall of the annular braking element has to be relatively thin. Therefore, such a braking element has a characteristic similar to that of a spring. Such a braking element is very sensitive to fluctuations in the diameter of the lead. Thus, the friction forces in the engagement between such a braking element and the lead may vary from one lead to another. Since the friction forces have to be overcome during the use of such a pencil, any increase of these forces may lead to hardening and roughening of the writing or drawing action. On the other hand, should the frictional forces be too small, then there is no guarantee that the lead is prevented from undesired sliding along the lead-advancing passage when the collet is open.
It has also been suggested to provide the tip of the mechanical pencil with a spring which normally urges the tip in direction outwardly of the casing. The tip, during the advancement movement causes a corresponding movement (i.e. slip) of the lead along the lead-guiding passage together with the tip. Thus, the length of the portion of the lead projecting beyond the tip includes a first portion achieved by the intentional advancement of the lead and a second portion stipulated by the controllable slip of the lead together with the tip. If the lead extends too far beyond the tip, it will easily break during writing or drawing. In order to avoid this, a user must adjust the position of the lead relative to the tip. Obviously, this additional adjustment involves additional unproductive time-consumption and is, generally-speaking, undesirably troublesome for the user.